Method for making a low avalanche voltage metal oxide varistor

ABSTRACT

A method of making low avalanche metal oxide varistors is described. Metal oxide powders are isostatically pressed to form a rod. The rod is fired, and then sliced into a plurality of discs. Electrodes are applied to the major faces of the discs, to produce discrete varistors. Thinner discs can be made by this technique than can be made by uniaxially pressing discrete discs, to get lower avalanche voltages. Also, comparative data is presented showing that varistors from slices of this isostatically pressed rod inherently have lower avalanche voltages than uniaxially pressed discrete disc varistors.

United States Patent 1191 Hossenlopp 1 51 May 27, 1975 METHOD FOR MAKINGA LOW AVALANCHE VOLTAGE METAL OXIDE VARISTOR [75] Inventor: Arthur M.Hossenlopp, Kokomo,

Ind.

[73] 'Assignee: General Motors Corporation,

Detroit, Mich.

221 Filed: Nov. 12, 1973 21 Appl. No.: 414,814

[52] US. Cl. 252/519; 29/621; 252/518 [51] Int. Cl. ..H01c 7/10; HOlc17/00 [58] Field of Search 252/518, 519; 29/610, 621

3,805,114 4/1974 Matsuoka et al. 252/518 X Primary Examiner-T. l-l.Tubbesing Attorney, Agent, or FirmRobert J. Wallace [5 7 ABSTRACT Amethod of making low avalanche metal oxide varistors is described. Metaloxide powders are isostatically pressed to form a rod. The rod is fired,and then sliced into a plurality of discs. Electrodes are applied to themajor faces of the discs, to produce discrete varistors. Thinner discscan be made by this technique than can be made by uniaxially pressingdiscrete discs, to get lower avalanche voltages. Also, comparative datais presented showing that varistors from slices of this isostaticallypressed rod inherently have lower avalanche voltages than uniaxiallypressed discrete disc varistors.

5 Claims, No Drawings METHOD FOR MAKING A LOW AVALANCHE VOLTAGE METALOXIDE VARISTOR BACKGROUND OF THE INVENTION This invention relates tometal oxide varistors and more particularly to an improved method thatis particularly useful in making metal oxide varistors that avalanche atlower voltages.

Varistors are resistors having markedly nonlinear voltage-amperecharacteristics. Resistivity varies with current and voltage. At somevoltage, for any given current level, the varistor becomes highlyconductive. This voltage is referred to herein as the avalanche voltage.In addition to current, avalanche voltage is also known to be affectedby metal oxide composition and thickness of the varistor. The oxidefiring process, of course, can affect it too. However, in general thereis a distinct increase in avalanche voltage per unit increase invaristor thickness for each distinct oxide composition used.

It is not especially difficult to make metal oxide varis ,tors whichavalanche above 100 volts. Such devices are conventionally made byuniaxially cold pressing a selected mixture of metal oxide powders intoa relatively thick disc, firing the disc, grinding or lapping the discto obtain the desired degree of flatness and parallelism on the oppositemajor surfaces of the disc, and then applying electrodes to these groundor lapped major surfaces. Each disc is thus separately formed and groundto a predetermined thickness, and then electrodes applied.

On the other hand, considerable difficulty lies in making varistors thatavalanche at less than 50 volts. Even with improved metal oxidemixtures, the discs must be made extremely thin. For example, discthickness must be reduced to thicknesses below 0.05 inch and even below0.03 inch. In some instances the discs simply cannot be made thin enoughto obtain the desired low avalanche voltage characteristic. It isextremely difficult to obtain uniform distribution of only a smallquantity of powdered materials over a relatively large area in apressing die. This produces an uneven density in the resultant thin discthat is formed, which in turn produces uneven avalanche characteristicsacross the face of the finished product. Analogously, poor surfaceflatness and parallelism can result in the disc after firing. Inaddition, the thinner the disc is, the more fragile it is. The fragilediscs must be handled both before and after firing, resulting inconsiderable waste due to breakage.

As a result, effort has been directed toward developing metal oxidecompositions which inherently avalanche at lower voltages per unitthickness. With such compositions the varistor discs can be made inlarger thicknesses, which reduce the fabrication problems previouslyreferred to. However, these improved compositions are still not goodenough to eliminate them. Such compositions are described in UnitedStates Pat. No. 3,663,348 Masuyama et al., as well as in the Journal ofthe ElectrochemicalSociety, Vol. 114, No. 8, pages 833-842, in anarticle entitled Polycrystalline Zinc Oxide Dielectrics, by Delaney etal.

On the other hand, I have found a new method of using thesecompositions, which avoids the fabrication problems referred to, andpermits one to readily and consistently obtain varistors with lowavalanche voltages.

OBJECTS AND SUMMARY OF THE INVENTION It is, therefore, a principalobject of this invention to provide an improved method of making metaloxide varistors, which is particularly useful in making metal oxidevaristors that avalanche at low voltages.

This object and other objects, features and advantages of this inventionare obtained by isostatically pressing a selected mixture of metal oxidepowders into an elongated body, firing the elongated body to form acoherent mass having a predetermined nonlinear voltagc-amperecharacteristic, transversely sawing the elongated body into a pluralityof thin slices having substantially flat and parallel opposite majorfaces as formed, and attaching electrodes to the opposite major faces ofthe slices.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In this invention, the variatordiscs are made by transversely slicing a tired bar, not by uniaxial coldpressing. The slices can readily be made much thinner than one can makediscs by cold pressing. Moreover, as formed, the slices are flat andparallel. Satisfactory devices can be made without extensive grinding orlapping, and in most instances it is unnecessary. Discs considerablythinner than 0.050 inch can readily be made. Hence, varistors having lowavalanche voltages can readily be made. in general, this technique ismost advantageous in making varistor discs below 0.05 inch in thickness,and more particularly below about 0.03 inch. Varistor discs of about0.01 4 0.02 inch in thickness can readily be made using equipment suchas used in sawing slices from monocrystalline semiconductor ingots. Eventhe thinnest discs are substantially flat and parallel as formed.

However, in addition to these and other benefits I have unexpectedlyfound still. another benefit. Testing has shown that relatively thickvaristors produced by my method inherently exhibit a lower avalanchevoltage than equally thick varistors produced by uniaxial pressing. Aswill subsequently be shown by comparative data, there is a significantdifference. Presumably, this advantage is present in my thin varistorstoo. Thus, a double advantage is apparently obtained when making thinvaristors by my method. Firstly, one can make thinner varistor discs, toinherently obtain lower avalanche voltages. Secondly, the resultant thindiscs apparently exhibit a lesser increase in avalanche voltage per unitincrease in thickness than uniaxially pressed discs.

The oxide mixture for my method is prepared for pressing in the same wayan oxide mixture is prepared for pressing in the conventional uniaxialcold pressing technique. Any of the known and accepted oxide pow dercompositions and preparation techniques can be used. However, my methodis directed to the manufacture of low avalanche voltage varistors. Iprefer to use those metal oxide compositions which are particularlyformulated to produce lower avalanche voltages. Zinc oxide mixturescontaining a small amount of bismuth oxide are known to produce thiseffect, and are generally preferred. It is all preferred that themixtures have a particle size which is less than about 20 microns. Onespecific oxide composition which can be used includes,

in mole percent, ZnO 98 percent, Bi O 0.5 percent, C 0 0.5 percent, MnO0.5 percent, and Cr O 0.5 percent.

ln an example of my process, 100.72 grams of the above specific oxidecomposition was placed with 300 cc. of methanol in a container, and ballmilled for three hours. Ball milling, rather than simple mixing, insuresthat the oxide particle size will be below about 20 microns and that themixture will be uniform. After milling, the batch was emptied onto asheet of aluminum foil and air dried. It was then passed through a 20mesh screen, which separated the mixture from the balls used during ballmilling. About grams of an aqueous solution containing 2 percent byweight polyvinyl alcohol inch and fired identically as described abovefor the isostatically pressed rod. After firing, the discs were groundand lapped in the normal and accepted manner, which reduced them to athickness of about 0. 145 inch. Electrodes were then applied to theirmajor faces in exactly the same way electrodes were applied to theaforementioned slices from the isostatically pressed rod.

The following table shows the results obtained on testing the varistorsthus made, with varistors 1 4 being the uniaxially pressed discs, andvaristors 5 7 being slices from the isostatically pressed rod. Theavalanche voltage is shown in volts to the right of each of the sevenvaristors for each of the current levels noted.

Varis- Microamperes Milliamperes tor .1 .2 .5 l 2 3 5 1O 40 was thenadded to the mixture and uniformly blended with it. The resultant powderblend was then passed through a 20 mesh screen and dried overnight at 65C.

The powder blend was then placed in an elongated rubber isostaticpressing mold, and the mold sealed. The interior of the mold wasgenerally cylindrical, having a diameter of about /2 inch and a lengthof about 3 inches. The mold was lowered into a pressing chamber of anisostatic pressing apparatus wherein the mold was surrounded by apressing fluid. The pressing chamber was then sealed, and the fluidpressurized to about 19,400 pounds per square inch, with a 5 minutedwell at that pressure. The pressure was then released, the pressingchamber opened, and the rubber mold removed. A compacted rod of theoxide blend was produced.

The rod was removed from the mold and heated in an oven at 220 C. forabout 40 hours. It was then placed in a cool furnace and furnace heatingstarted. When the furnace reached a soak temperature of 2,300 F., it washeld there for about three hours. Furnace heating was then discontinuedand the furnace allowed to cool overnight, about 15 hours, to atemperature of about 300 F. 400 F. During the first hour the furnacecooled about 300 F. 500 F. The rod was then removed from the furnace andallowed to cool to room temperature. It was then transversely slicedinto a plurality of discs about 0.145 inch in thickness, with the majorsurfaces of the slices being substantially flat and parallel as sliced.

The slices were then ready for application of electrodes to their majorsurfaces without any further operations such as grinding or lapping. Theelectrodes were applied by hand brushing a commercially availablesilver-palladium-glass cermet mixture onto their major surfaces, andthen fusing the mixture to bond it to the slice.

For comparison, an identical powder blend was prepared as describedabove and used to make 0.58 inch diameter circular discs by uniaxialcold pressing. The discs were individually pressed to a thickness of0.156

All seven of the above varistors were nominally of the same size.However, as can be seen from the above table, the uniaxially pressedvaristors consistently avalanched at higher voltages than varistors madefrom slices of the isostatically pressed rod. There is a difference at acurrent level of one milliampere of about one volt per 0.001 inch invaristor thickness. Alpha, the change in voltage with current, wasgenerally higher in varistors made from the isostatically pressed bar.

As mentioned, in my method the oxide powder is prepared for pressing inthe known and accepted manner. For example, if the particle size of thepowdered mixture is not of a small enough size, it is initially groundto a smaller particle size. A temporary binder and lubricant is normallyincluded in the mixture. In the examples described above, polyvinylalcohol was used both as a temporary binder and lubricant. About 2 5percent by weight of the 2 percent aqueous solution can be used.However, other substances can be used for such a combined function, orseparate substances can be used for each.

Similarly, after blending with additives, the powdered mixture isscreened and dried to form agglomerates of appropriate size forpressing. The size of the agglomerates used, dryness, and manner offormation are all variables, as they are in uniaxial cold pressing.

My isostatically pressed rod can be fired in the known and accepted wayfor uniaxially pressed discs. By this 1 mean it should be preheated,raised to a high temperature, and slowly cooled. However, the precisepreheating and slow cooling used in my method is more critical than itis in firing uniaxially pressed discs. For example, the rod can beinitially slowly heated, or heated to some constant lower temperaturefor a period of time, to drive off volatiles such as water, thetemporary binder, and lubricant. Then it can be heated to the maximumfiring, or soak, temperature. Analogously, it is not necessary to placethe pressed rod in a cool furnace and then start heating to the desiredsoak temperature. However, such practice will insure that any remainingvolatiles can escape without adversely affecting the pressed rod. Aftersoaking at the desired temperature the pressed part is then slowlycooled. Furnace cooling is a typical technique by which this can bedone.

As is known, the specific soak temperature and time for soaking usedduring the firing will affect the electrical properties and theirresultant product. I prefer to use a soak temperature of approximately2,000 F. 23,00 F. for about 1 3 hours. At the higher temperature Iprefer to soak for only about one hour, while at the lower temperature Iprefer to soak about three hours. However, it is to be understood thatthe particular time and temperature used will depend upon the particularelectrical characteristics desired.

The particular form of isostatic pressing which can be used is notespecially significant. The technique previously described herein, a wetbag" technique, can be employed as well as the dry bag technique whichis disclosed in US. Pat. No. 3,034,191 Schaefer. The pressure usedduring the isostatic pressing is preferably about 15,000 to 20,000pounds per square inch. However, in some instances one may desire to usea pressure as low as only 10,000 pounds per square inch.

The fired bar is sliced in my method using a diamond saw to obtain thenarrowest saw width, and least kerf waste. However, other saws such as asilicon carbide saw could be used. The thickness of the slice sawed willdepend upon the selected avalanche voltage which is desired. Thegeneral, it is most advantageous to use my method in making discvaristors less than about 0.5 inch thick.

I claim:

1. A method of making nonlinear metal oxide resistors with low avalanchevoltage characteristics comprising the steps of:

forming a uniform mixture of nonlinear resistor metal oxide powders forpressing into a predetermined shape,

isostatically pressing said mixture under a pressure of about 10,00020,000 pounds per square inch to form an elongated body,

firing said elongated body at a temperature of about 2000 C. to 2300 C.for about 1 3 hours to produce a coherent mass having a predeterminednonlinear voltage-ampere characteristic,

transversely sawing said elongated body into a plurality of thin discs,each of which has substantially flat and parallel opposite major facesas formed that are suitable for attachment of electrodes and has athickness less than about 0.05 inch commensurate with a predeterminedavalanche voltage desired, and

attaching electrodes to said opposite major faces of said discs to forma plurality of discrete nonlinear metal oxide resistors that avalancheat a predetermined voltage below about 50 volts. 2. A method of makingnonlinear metal oxide resistors with low avalanche voltagecharacteristics comprising the steps of:

uniformly mixing a plurality of nonlinear resistor metal oxides having aparticle size less than about microns,

isostatically pressing said oxides under a pressure of about 15,00020,000 pounds per square inch to form an elongated body,

firing said elongated body at a temperature of about 2000 C. 2300 C. forabout 1 3 hours,

sawing said body into aplurality of discs having substantially flat andparallel opposite major surfaces that as formed are suitable forattachment of electrodes and having a thickness less than about 0.05inch commensurate with a predetermined low avalanche voltage desired,and

attaching electrodes to said opposite major faces of said discs to forma plurality of discrete nonlinear metal oxide resistors that avalancheat a predetermined voltage below about 50 volts.

3. A method of making nonlinear metal oxide resistors with low avalanchevoltage characteristics comprising the steps of:

uniformly mixing zinc oxide and a small proportion of other metal oxideswith a temporary binder, said zinc oxide and other oxides having aparticle size less than about 20 microns,

isostatically pressing said uniform mixture under a pressure of about15,000 20,000 pounds per square inch to form an elongated body,

firing said elongated body at a temperature of about 2,000 C. 2,300 C.for about 1 3 hours to produce a coherent mass having a predeterminednonlinear voltage-ampere characteristic, transversely sawing said bodyinto a plurality of discs having substantially flat and parallelopposite major faces as formed that are suitable for attachment ofelectrodes and having a thickness of less than 0.03 inch commensuratewith a predetermined low avalanche voltage desired, and

attaching electrodes to said opposite major faces of said discs to forma plurality of discrete nonlinear metal oxide resistors that avalancheat a predetermined voltage below about 50 volts. 4. A method ofconsistently and reliably making metal oxide varistors with extremelylow avalanche voltages comprising the steps of:

forming a uniform mixture of a plurality of metal oxide powders having aparticle size less than about 20 microns and 2 5 percent by weight of anaqueous solution containing 2 percent by volume polyvinyl alcohol, saidplurality of metal oxide powders consisting essentially of, by molepercent, about 98 percent ZnO. 0.5 percent Bi O 0.5 percent C0 0 0.5percent MnO, and 0.5 percent Cr O passing said uniform mixture through ascreen to obtain mixture agglomerates of desired size, drying thescreened agglomerates, isostatically pressing the dry powderagglomerates into an elongated generally cylindrical body at about15,000 20,000 pounds per square inch,

preheating said body to remove said polyvinyl alcohol from said mixture,

heating said body to a temperature of about 2,000

C. 2,300 C. and maintaining said body at said temperature for about 1 3hours and then allowing said body to slowly cool,

transversely sawing said body into a plurality of discs havingsubstantially flat and parallel opposite major faces as formed that aresuitable for attachment of electrodes and having a thickness less thanabout 0.03 inch commensurate with a predetermined desired nonlinearvoltage-amphere characteristic, and

attaching an electrode to each of said opposite major faces of saiddiscs to form a plurality of discrete resistor discs having a nonlinearvoltage-ampere characteristic with an avalanche voltage below about 50volts.

5. A metal oxide varistor made in accordance with the method as recitedin claim 1.

1. A METHOD OF MAKING NONLINEAR METAL OXIDE RESISTORS WITH LOW AVALENCHEVOLTAGE CHARACTERISTICS COMPRISING THE STEPS OF: FORMING A UNIFORMMIXTURE OF NONLINEAR RESISTOR METAL OXIDE POWDERS FOR PRESSING INTO APREDETERMINED SHAPE, ISOSTATICALLY PRESSING SAID MIXTURE UNDER APRESSURE OF ABOUT 10,000 - 20,00O0 POUNDS PER SQUARE INCH TO FORM ANELONGATED BODY, FIRING SAID ELONGATED BODY AT A TEMPERATURE OF ABOUT2000* C. TO 2300*C. FOR ABOUT 1 - 3 HOURS TO PRODUCE A COHERENT MASSHAVING A PREDETERMINED NONLINEAR VOLTAGE-AMPERE CHARACTERISTIC,TRANSVERSELY SAWING SAID ELONGATED BODY INTO A PLURALITY OF THIN DISCS,EACH OF WHICH HAS SUBSTANTIALLY FLAT AND PARALLEL OPPOSITE MAJOR FACESAS FORMED THAT ARE SUITABLE FOR ATTACHMENT OF ELECTRODES AND HAS ATHICKNESS LESS THAN ABOUT 0.05 INCH COMMENSURATE WITH A PREDETERMINEDATTACHING ELECTRODES TO SAID OPPOSITE MAJOR FACES OF SAID DISCS TO FORMA PLURALITY OF DISCRETE NONLINEAR METAL OXIDE DISCS TO FORM A PLURALITYOF DISCRETE NONLINEAR METAL OXIDE RESISTORS THAT AVALENCHE AT APREDETERMINED VOLTAGE BELOW ABOUT 50 VOLTS.
 2. A method of makingnonlinear metal oxide resistors with low avalanche voltagecharacteristics comprising the steps of: uniformly mixing a plurality ofnonlinear resistor metal oxides having a particle size less than about20 microns, isostatically pressing said oxides under a pressure of about15, 000 - 20,000 pounds per square inch to form an elongated body,firing said elongated body at a temperature of about 2000* C. -2300* C.for about 1 - 3 hours, sawing said body into a plurality of discs havingsubstantially flat and parallel opposite major surfaces that as formedare suitable for attachment of electrodes and having a thickness lessthan about 0.05 inch commensurate with a predetermined low avalanchevoltage desired, and attaching electrodes to said opposite major facesof said discs to form a plurality of discrete nonlinear metal oxideresistors that avalanche at a predetermined voltage below about 50volts.
 3. A method of making nonlinear metal oxide resistors with lowavalanche voltage characteristics comprising the steps of: uniformlymixing zinc oxide and a small proportion of other metal oxides with atemporary binder, said zinc oxide and other oxides having a particlesize less than about 20 microns, isostatically pressing said uniformmixture under a pressure of about 15,000 - 20,000 pounds per square inchto form an elongated body, firing said elongated body at a temperatureof about 2,000* C. -2,300* C. for about 1 - 3 hours to produce acoherent mass having a predetermined nonlinear voltage-amperecharacteristic, transversely sawing said body into a plurality of discshaving substantially flat and parallel opposite major faces as formedthat are suitable for attachment of electrodes and having a thickness ofless than 0.03 inch commensurate with a predetermined low avalanchevoltage desired, and attaching electrodes to said opposite major facesof said discs to form a plurality of discrete nonlinear metal oxideresistors that avalanche at a predetermined voltage below about 50volts.
 4. A method of consistently and reliably making metal oxidevaristors with extremely low avalanche voltages comprising the steps of:forming a uniform mixture of a plurality of metal oxide powders having aparticle size less than about 20 microns and 2 - 5 percent by weight ofan aqueous solution containing 2 percent by volume polyvinyl alcohol,said plurality of metal oxide powders consisting essentially of, by molepercent, about 98 percent ZnO. 0.5 percent Bi2O3, 0.5 percent Co2O3, 0.5percent MnO, and 0.5 percent Cr2O3, passing said uniform mixture througha screen to obtain mixture agglomerates of desired size, drying thescreened agglomerates, isostatically pressing the dry powderagglomerates into an elongated generally cylindrical body at about15,000 - 20,000 pounds per square inch, preheating said body to removesaid polyvinyl alcohol from said mixture, heating said body to atemperature of about 2,000* C. - 2,300* C. and maintaining said body atsaid temperature for about 1 -3 hours and then allowing said body toslowly cool, transversely sawing said body into a plurality of discshaving substantially flat and parallel opposite major faces as formedthat are suitable for attachment of electrodes and having a thicknessless than about 0.03 inch commensurate with a predetermined desirednonlinear voltage-amphere characteristic, and attaching an electrode toeach of said opposite major faces of said discs to form a plurality ofdiscrete resistor discs having a nonlinear voltage-ampere characteristicwith an avalanche voltage below about 50 volts.
 5. A metal oxidevaristor made in accordance with the method as recited in claim 1.